The present invention relates generally to variable spot size illuminators, and more particularly, to such illuminators that can provide a homogeneous illumination spot whose size can be continuously varied while exhibiting parfocality.
A number of ophthalmic surgical procedures performed on a patient's retina require illuminating a selected portion of the retina with an illumination spot, typically provided by a laser, having a desired size. For example, in one such surgical procedure, known as photocoagulation, a laser light spot is directed to a selected portion of a patient's retina to cause coagulation of the illuminated tissue by generating heat. Photocoagulation can be employed, for example, to seal leaky blood vessels, destroy abnormal blood vessels, or heal retinal detachment.
In another ophthalmic surgical procedure, commonly known as photodynamic therapy, an agent, which is inert in the absence of light activation, is initially administered intravenously to the patient. Subsequently, abnormally highly vascularized retinal tissue containing the agent is illuminated with laser light having a selected wavelength to activate the agent. The activated agent can destroy the abnormal tissue or have other beneficial therapeutic effects.
In such procedures, it is generally advantageous that the light intensity over the illuminated area be substantially uniform, and remain stable for the illumination period. Further, the size of the illumination spot may need to be varied while ensuring that the location of the spot remains focused on the patient's retina. In practice, a surgeon typically employs an illuminator for performing an ophthalmic procedure together with an observation system, such as a slit lamp microscope or an indirect ophthalmoscope, that allows the surgeon to observe and treat a desired area. The focus associated with the illuminator should coincide with the focus associated with the observation system so that the surgeon can simultaneously observe and treat a desired area. That is, it is desirable that the illuminator and the observation system be parfocal. In general, two independent optical systems with foci that lie on the same focal plane are known as being parfocal, and this relationship is known as parfocality. Traditional variable spot size illuminators provide variable magnification of a light spot formed on a treatment plane, e.g., a patient's retina, by moving one or more lenses in a manner that causes the movement of the illuminator's focal plane. Thus, in traditional variable spot size illuminators, although the illuminator may be parfocal with an observation system at one spot size, the parfocality is lost at a different spot size. This in turn requires the surgeon to refocus or re-accommodate at different spot sizes, and further adversely affects the image quality, e.g., sharpness of focus, of the treatment spot.
Further, a patient undergoing laser treatment is not entirely motionless. In fact, the patient's head typically move, for example, back and forth during the treatment procedure. In most conventional slit lamp microscopes, the surgeon can displace the slit lamp's observation and treatment optics to follow the patient's motion in order to ensure that the patient's retina remains aligned and in-focus relative to the aiming beam. While a patient's sideways displacements can be readily tracked through the displacement of the aiming beam on the retina, the patient's displacement along the optical axis, that is, towards and away from the observation/treatment optics, can typically only be tracked by the surgeon's perception of the sharpness of the focus of the retinal image and of the aiming beam spot. Such perceived sharpness of focus is, however, often misleading because of differences in depth of focus at different magnifications of the observation system, and variability in the ability of different surgeons to accommodate. This can result in a considerable variation of the treatment beam's spot size, and consequently the treatment beam's intensity, on the retina over the range of the slit lamp's depth of focus.
Accordingly, there is a need for medical illuminators that can provide a spatially homogeneous illumination spot on a selected portion of a patient's retina.
There is also a need for such illuminators that can allow readily adjusting the size of the illumination spot on a selected portion of the patient's retina.
Moreover, there is a need for such illuminators that allow adjusting the size of an illumination spot while ensuring that the illuminator's focal distance, known also as the working distance, remains substantially constant, thereby maintaining parfocality with other optical systems coupled to the illuminator. In other words, there is a need for illuminators that exhibit “spot size variation parfocality.”
Further, there is a need for medical illuminators in which the spot size and the intensity profile of a treatment laser remain substantially stable, e.g., they vary by less than about 10%, over a selected distance (e.g., mm) from the laser's focal plane. In other words, there is a need for medical illuminators that exhibit “observation depth of focus parfocality” in which the treatment conditions are reasonably stable through the entire range of the observation system's depth of focus.